This research program is designed to investigate the structure and enzymatic properties of myosin in pregnant and non-pregnant uterus. The hypothesis to be investigated is the presence of "pregnancy specific" genes in uterus that are specifically expressed during pregnancy, thus structure of myosin (and other muscle proteins) would be different in pregnant and non-pregnant uteri. Differences in myosin structure would cause differences in myosin enzymatic properties and in the mechanism of actin-myosin interaction which in part should be responsible for the changes of uterine contractility properties during pregnancy. The overall objectives of this project include comparative studies of myosin in non-pregnant and pregnant uterus: a, subunit structure; b, molecular weight and charge properties of the myosin light chains; c, isolation and sequencing of homologous active-site peptides of myosin; d, investigation of ATP-ase activity of the myosin before and after chemical modifications. Other studies on human placental myosin and actin-myosin interactions are closely related to the structural work. They explore the problems of the regulation of contraction-relaxation cycle in the contractile elements of the utero-placental system.